1. Field of the Invention
This invention relates to the field of air flow control. More particularly, it relates to a variable air volume (VAV) box used to control the flow of air by volume through the box and associated ductwork.
2. Description of the Prior Art
VAV boxes are used in air conditioning systems to adjust and control the supply of air to spaces within a building. VAV boxes have therefore essentially one basic function, i.e., to control air volume. Additions to that configuration include positioning coils in the air flow path to moderate air temperature. The basic box is provided in varying sizes depending upon the requirements of the application. A single blade control element having a square or rectangular shape is mounted for rotation to control the volume of air flow through the box.
The walls of a conventional VAV box are made of flat, straight sections of sheet metal that offer no advantage to air control characteristics.
A VAV box is fully closed when its blade or blades are in their respective non-rotated positions, i.e., when each blade has zero degrees (0°) of rotation. A VAV box is fully open when each blade is rotated ninety degrees (90°) relative to its fully closed position. The relation between volume of air flow and blade position in a conventional single blade control element is non-linear.
More particularly, conventional single blade control elements exhibit an increased air flow at the beginning of blade rotation, i.e., as the blade starts to open. For example, a zero to ten degree (0°-10°) of rotation of the blade corresponds to about a thirty percent (30%) increase in air flow. A fifty-five to ninety degree (55°-90°) of rotation corresponds to only a ten percent (10%) increase in air flow.
This non-linear characteristic gives rise to numerous control problems. Air passing through a VAV box and the air space temperature are difficult to control because the relationship of blade position to necessary control parameters are non-linear. Control feedback loop response times are also increased. Pressure downstream of the VAV box becomes variable, and calculation of minimum and maximum air volume to obtain a valid air balance becomes nearly impossible. Historically these problems have been addressed in numerous and unsuccessful ways.
Adjustment of the linkage that connects the actuator to the blade has been tried in various forms. Such adjustment produces a variable “exchange” ratio between the actuator and blade position. Linkage-to-actuator inaccuracies and tolerances lead to “play” in that connection, which is critical, especially in the first stages of opening. A small degree of play in the initial stages of opening has great impact on flow volume characteristics.
Another attempt to produce linear air flow includes programming the output signal to an actuator associated with a particular VAV box. After programming, the output of the actuator becomes non-linear in such a way that it causes air flow through the VAV box to become linear. However, this assumes that the programmer knows the exact characteristics of the specific VAV box in use, and the needs of the space that it serves, which assumption is usually invalid.
These problems have historically made it hard to accurately control air flow through a standard VAV box to achieve an accurate air balance.
However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in this art how the identified needs could be met.